Re: Sky Brigntness in I

["Richard Miles" <rmiles.btee@btinternet.com>]

>I was just looking at the .list files from a few minutes ago.  I was
> concerned that the tom3 v camera reconstructed every image whie the i
> image had no successes.  This for 19 v and 19 i images.  On inspection,
> all the i images had excess sky brightness while the v images were
> normal.
>
> Looking further:
>
> tom1 looking near horizon v and i background about 5000 ADU.
> tom2 looking straight up v of order 10000 counts, i of order 15,000
> counts.
> tom3 looking toward pole v of order 5000 counts, i of order 15,000
> counts.
>
> There is a full moon, but it should nearly be set for this data. Does
> anyone know about the brightness of the i sky?  I would have expected
> the camera looking straight up to have low sky brightness.

Tom,

I operate simultaneous V and I scopes and find that they are particularly 
effective near the horizon: altitudes of 5-20 deg.  This is because of 
several factors; most importantly lower attenuation of the *transmitted* 
light in the I band.

If you take the situation looking towards the zenith with a nearly full Moon 
low in the sky, then if there is a small amount of mist/cirrus cloud around, 
you will be in the somewhat opposite situation.  What I find is that the I 
image is very sensitive to reflected light from mist / cloud: here you are 
seeing *reflected* I-band light producing the high sky brightness, and this 
will be relatively intense compared to *reflected* V-band light.  In my case 
the V-band image is about a factor of 3 more efficient than the I-, so the 
signal-to-noise of stellar V images is also enhanced relative to I-, so the 
overall result is a very significant degradation of the I-band relative to 
the V-band given these atypical sky conditions.

There is one other difference  worth remarking on between the two filter 
bands when working with field sizes in the 1-4 degree range.  That is when 
there is trace of (usually higher altitude) cloud around.  You will see a 
lot more *structure* in the attenuating cloud in I-band than in the V-.  So 
this has a relatively deleterious effect on systematic biases between comp 
and target stars compared to the V- case.

If the sky is extremely clear and transparent then all these differences are 
much less marked.

Hope this helps,
Richard